I just make my own out of some ext or marine ply.
Either copy sizes from a makers site or design it to get most from a 8x4 sheet. i think, Usually takes 2 plus a thinner sheet, i insulate mine so have a thin 2-3mm ply on inside. My dogs seem quite happy in them
Not sure wot size mine are but usually 4-6" narrower than 4ft so u have plenty overhang on roof (2-3" each side). My boxes just sit out in covered runs. My oldest box could be 10+ years old and still looking ok.
I leave the roofs flat so dogs can jump up and sleep on them, my springer hardly sleeps inside the box unless very cold or wet/windy